In general, the present invention relates to a gas generator employed in a vehicle occupant protection system. As such, a preferred use of the present invention relates to a gas generator that may be typically used to activate a seatbelt pretensioner, for example.
Gas generators used in seatbelt pretensioners are known as micro gas generators given the relatively smaller size of the gas generator. Exemplary pretensioners include those described in U.S. Pat. Nos. 5,397,075 and 5,899,399, herein incorporated by reference.
Micro gas generators generally contain an initiator, an initiator retainer, a propellant cup, and a propellant contained within the propellant cup. In many micro gas generators, the propellant is provided in tablet form wherein the aggregate combustion surface area is substantially higher than in gas generants provided in monolithic gas generating casts, for example.
The total surface area of the propellant in any gas generator is often optimized based on the burning rate of the respective gas generant composition. Historically, many azide-based compositions have been satisfactorily provided in tablet form given the relatively low burn rate of many azide-based compositions. With the advent of non-azide compositions, however, relatively higher burn rates often require a relatively lower aggregate propellant surface area. Therefore, monolithic casts can be formed that effectively reduce the total propellant surface area exposed to combustion.
Some pretensioner designs may benefit from a relatively quick gas dump or production. However, most designs benefit from a relatively steady gas delivery such as produced by neutral burning surface shapes like perforated cylindrical propellant grains. An example is a typical state of the art micro gas generator (MGG) containing a non-azide fuel such as nitrocellulose, which produces the tank curve shown in FIG. 6. One disadvantage with the use of nitrocellulose is the excessive carbon monoxide produced upon combustion. Alternative propellant compositions improve upon the effluent and stability requirements, but have relatively high burn rates that pragmatically inhibit the use of small granules, small tablets, or small grain size.
Another problem with the use of granulated or small extruded propellant grains such as tablets is the potential release of the pyrotechnic material into the pretensioner once combustion begins. This may lead to the release of burning propellant from the pretensioner or burning of the pretensioner components. Retention of the propellant within the propellant cup during combustion would prevent these potential drawbacks.
When forming a monolithic propellant cast, the shape of the cast must be closely equivalent to the propellant cup. The failure to closely conform the outer surface of the propellant cast to the interior surface of the propellant cup detrimentally affects the predetermination of the total combustion surface area. Stated another way, any portion of the exterior surface of the cast that is not flush against the interior surface of the propellant cup presents additional surface area and thereby skews the combustion profile of the gas generator. As a result, predictable performance of the gas generator and/or the pretensioner becomes less probable.
Additionally, it is well recognized that the monolithic cast must have a well-mixed and homogeneous consistency throughout the cast. Failure to adequately mix the various gas generant constituents into a homogeneous mixture may result in incomplete combustion and/or inconsistent combustion profiles relative to operation of a seatbelt pretensioner, for example. In essence, an incomplete mix results in a heterogeneous mix of the fuel and oxidizer thereby resulting in incomplete combustion systems in certain regions of the solid propellant. One related concern is that well-mixed homogeneous compositions require more labor intensive or more complicated processes. In general, as the manufacturing process becomes more complicated, the production costs become substantially greater.
A gas generator containing a monolithic propellant cast solves the above-referenced problems. In accordance with the present invention, preferred gas generant constituents are combined within the propellant cup, wherein ammonium perchlorate or any other oxidizer is simply added to uncured silicone. Uncured curable silicone is first added to a propellant cup on the assembly line at about 15-30%% by weight of the total gas generant composition. Ammonium perchlorate is then added to the uncured silicone at 70-85% by weight of the gas generant composition. A preferred composition contains 27% by weight of silicone and 73% by weight of ammonium perchlorate. The present inventors have unexpectedly discovered that simply adding a granulated oxidizer to uncured silicone results in a homogeneous dispersion of the oxidizer throughout the silicone. On average, the oxidizer is homogeneously suspended within the silicone within one to two hours depending on the resin employed.
It is believed that the uncured silicone possesses dispersant properties as it cures, and thus facilitates the homogeneous dispersion of the ammonium perchlorate, or any other gas generant constituent known to be used in vehicle occupant protection systems, before the silicone is completely cured. As a result, actual mixing is not required and processing is therefore simplified. Furthermore, as the uncured silicone begins to cure, it adheres to the interior surface of the propellant cup thereby ensuring the outer surface of the cast conforms exactly to the inner surface of the propellant cup in flush communication therewith. Predetermined surface area is thus ensured, minimizing deviation in the average combustion profile. Furthermore, the monolithic cast is better retained within the propellant cup once combustion is initiated.